The human body is roughly 50 to 75% water by weight, with most adults falling somewhere around 60%. For a 155-pound person, that translates to about 10 to 11 gallons of water distributed across every organ, tissue, and cell. The exact percentage depends heavily on your age, sex, and how much body fat you carry.
What Determines Your Body’s Water Content
The 50 to 75% range is wide because body composition varies so much from person to person. Fat tissue holds considerably less water than lean tissue, so two people who weigh the same can have very different total body water. Women generally carry a lower percentage of water than men, largely because women tend to have a higher proportion of body fat. As people age, body water drops as well: total body water declines by roughly 3.7% between the ages of 20 and 70, partly because lean muscle mass decreases over time.
Newborns sit at the high end of the spectrum, with bodies that are about 75% water. By the time a child reaches their first birthday, that number has already started to fall. A typical adult man averages around 60%, while a typical adult woman averages closer to 50 to 55%. Older adults trend toward the lower end as muscle gives way to fat and other age-related changes take hold.
Water Content of Individual Organs
Not every part of you holds water equally. Some organs are surprisingly water-dense:
- Lungs: about 83% water, the highest of any major organ
- Muscles and kidneys: about 79% water
- Brain and heart: about 73% water
- Skin: about 64% water
- Bones: about 31% water, even though they feel completely solid
Blood plasma, the liquid portion of blood that carries red and white blood cells, is 90% water. Even tissues that seem dry or rigid contain meaningful amounts of water, because water is essential to the chemical reactions happening inside every cell.
Where the Water Actually Sits
Your body’s water isn’t just sloshing around freely. It’s divided into two main compartments. About two-thirds of total body water sits inside your cells, where it serves as the medium for all the biochemical work that keeps you alive. The remaining third is outside the cells, filling the spaces between tissues, flowing through blood vessels, and cushioning organs. The ratio between these two compartments stays remarkably stable in healthy people, with the outside-to-inside ratio hovering around 0.56.
This balance matters. When the ratio shifts, whether from dehydration, kidney problems, or certain diseases, your body struggles to regulate blood pressure, deliver nutrients, and clear waste. The distribution of water is just as important as the total amount.
What All That Water Does
Water is the solvent your body runs on. Nearly every process in your body either uses water directly or depends on water to function. It carries dissolved nutrients through the bloodstream, flushes waste products through the kidneys, cushions the brain and spinal cord, and lubricates joints so they move without friction.
Temperature regulation is one of water’s most critical jobs. When your body heats up during exercise or in a hot environment, it pushes blood toward the skin and produces sweat. As that sweat evaporates, it pulls heat away from your body. The more intense the exercise or the hotter the environment, the more your body depends on this evaporative cooling. Losing even a small percentage of your body water through sweat impairs this system: your sweating rate drops, blood flow to the skin decreases, and your core temperature climbs faster than it should. At the same time, less fluid in your blood vessels makes it harder for your heart to maintain adequate blood flow to both your muscles and your skin simultaneously.
How Much Water You Lose Each Day
An average adult loses about 2,400 milliliters (roughly 2.5 quarts) of water per day under normal conditions, even without heavy exercise. The biggest single route is urine, which accounts for about 1,500 milliliters. Another 900 milliliters escapes through “insensible” losses, meaning water that evaporates from your skin and leaves your lungs every time you exhale. You don’t notice these losses because they happen continuously and invisibly. A smaller amount leaves through the digestive tract.
These numbers shift dramatically with physical activity, heat exposure, illness, or altitude. A person exercising hard in the heat can lose more than a liter of sweat per hour, pushing total daily water loss well above the baseline. Your body replaces this water through drinking, eating (most foods contain a surprising amount of water), and a small amount generated internally when your cells break down nutrients for energy.
How Body Fat Changes the Math
If there’s one factor that explains most of the variation in body water percentage, it’s body fat. Fat cells contain very little water compared to muscle cells. Someone with a lean, muscular build will have a total body water percentage toward the upper end of the range, while someone carrying more body fat will be closer to 50%. This is the main reason the “60% water” figure often cited as a universal average can be misleading. It’s a rough midpoint, not a fixed number that applies to everyone.
This also explains why hydration assessments based solely on the ratio of water to body weight are unreliable. Two people can be equally well-hydrated but show very different percentages simply because of differences in body composition.